in the hole

(stud poker) Of a card ("the hole card") which is dealt face down and thus unknown to all but its holder; hence (idiomatic) in reserve; in particular ace in the hole.

Example: 1993, Shelby Foote, introduction to Stephen Crane, The Red Badge of Courage, The Modern Library (1993), ISBN 9780679641292, unnumbered pages:
  Further testimony to his commercial incompetence lay in the fact that, at fifty cents a copy, a complete sellout would have left him worse than $300 in the hole—proof enough, if proof was needed, that it wasn't primarily money he was after; it was fame.
1999, Dwight Ott, "A Share Of Bridge Tolls For Camden?", Philadelphia Inquirer, 18 June 1999:
  Finance Director Robert Law added that even if Camden laid off all of its civilian employees, it would still be millions in the hole.
2011, Matt Prigge, "How 'Drive' Director Nicolas Winding Refn was influenced by 'Pretty Woman'", Philadelphia Weekly, 15 September 2011:
  Instead it was a critical and commercial bomb that put his production company millions in the hole, necessitating two Pusher sequels (in 2004 and 2005) to get him in the black.